In view of the ever increasing production volumes in most industrial branches much attention is being devoted lately to environmental protection, the more stringent requirements being imposed upon separating from sewage water industrial waste products which include solid matter and dissolved harmful substances.
One important waste water purification process stage involves recovery of solid phase from suspension by filtration.
Vacuum filters and centrifuges are generally employed for filter-separation of easily filterable suspensions having a rather low hydraulic resistance. For filtering suspensions that are more difficult to filter, that is suspensions processing a rather high hydraulic resistance where use in normally made of filter presses.
Modern developments in the art of filter construction have taken special care to provide a larger filtering surface within one unit used in industrial fields where large bodies of suspensions have to be treated. This has been caused by the fact that the use of a multitude of small filtering surface filter presses result in excessive specific manufacturing and operating expenditures per unit area of the filtering surface. The expenditures thus encountered are accounted for by the fact that each small capacity filter press is to be equipped with independent automatic process control units, sets of valving fixtures, and other auxiliaries requiring numerous attending personnel.
It is to be noted that an increase in the filtering surface entails, due to employing a greater number of filter plates in the filter press, a problem associated with assuring a reliable hermetization of the filter plate assemblies.
However, no satisfactory solution to these problems has been found in the prior art filter press constructions.
Major demands placed on filter press constructions include the provision of high filtering capacity, as well as obtaining filter cakes of minimized moisture content and in some instances of minimal content of substances extractable from the cake inside the filter press.
There is known a filter press which includes a frame mounted on posts and accommodating an assembly of filter plates with a means for clamping or compressing these filter plates (cf., West German Application No. 2,707,828; published 1978).
The means for clamping the filter plate assembly includes a drive secured on the posts at one end of the frame, a thrust plate also arranged on the posts at the opposite end of the frame, a pressure plate, and braces. The filter plate assembly is interposed between the pressure and thrust plates.
The filter plates are connected therebetween and with the thrust and pressure plates by linkages providing for spacing all the plates at substantially equal distances, which enables to remove filter cake. The braces connect the thrust plate and the drive of the means for clamping the filter plate assembly.
In the course of clamping the filter plate assembly the drive of the clamping means acts on the pressure plate to compress the filter plates between the pressure and thrust plates. Therewith, the braces connecting the thrust plate and the drive of the means for clamping the filter plate assembly take up the compressive force to thereby assure hermeticity of the filter plate assembly.
For taking off the filter cake the means for clamping the filter plate assembly is engaged which acts to withdraw the pressure plate and space all the filter plates at equal distances, whereafter the cake is removed.
In the above filter press the movable member of the drive for clamping the filter plate assembly has a limited length determined by necessary strength characteristics. Therefore, the work stroke of the movable member of this means is limited by its size, which in turn leads to restrictions in the number of filter plates in the assembly, since the total length of spaces between the plates must not be in excess of the stroke length of the movable member of the drive.
There is also known a filter press comprising a frame mounted on posts and accommodating an assembly of filter plates, and a means for clamping the filter plates (cf., West German Utility Model No. 8,025,470; priority of Sept. 23, 1980).
The means for clamping the assemblies of filter plates in this press filter comprises a thrust plate secured on the posts at one end of the frame, a drive of the means for clamping the filter plate assemblies, a pressure plate disposed therebetween, and braces in the form of substantially flat beams arranged on the sides of the filter press in parallel with its longitudinal centerline and affixed by ends to the frame posts occupying the opposite end of the frame. The drive of the means for clamping the filter plate assembly is disposed on the frame for displacement along said braces and provided with retractable pins. When retracted, these pins cooperate with the braces by introducing them into holes made in the brace structure for fixing and thrusting the drive for clamping the filter plate assembly prepared for clamping.
For effecting reciprocating movement of the drive for clamping the filter plate assemblies it is provided with rollers and connected to a drive imparting movement thereto along the braces.
In order to assure hermeticity of the filter plate assembly, the drive for clamping the filter plates and the pressure plate are displaced toward the filter plate assembly, the position of the drive being fixed in the immediate proximity to the assembly by means of the retractable pins and holes made in the braces. Thereafter, the drive for clamping is actuated for the movable member thereof to compress the filter plate assembly and assure its hermeticity. The retractable pins provide for the thrust of the housing of the drive in the holes of the braces.
After terminating the filtration process the drive for clamping the filter plate assembly is disengaged, the retractable pins are withdrawn from the holes in the braces, and the drive for clamping the filter plate assembly together with the pressure plate is drawn away from the assembly by means of the longitudinal movement drive. Then the filter cake is removed from the filter press.
However, in the course of operation of the above filter press it becomes necessary to replace the filter cloth the thickness of which for various types of suspensions may vary within a wide range. This may cause substantial variations in the overall length of the filter plates (between 100 and 300 mm). Because the holes in the braces serving for driving thereinto the pins of the drive are made in strictly predetermined locations, and the fixation of the drive is possible only in these holes, then for fixing the drive in a working position in some instances it is necessary to remove 2 to 4 filter plates, which affects the efficiency of the filter press.
In addition, it also has to be noted that it is not always possible to increase the filtering surface of such a filter press by merely increasing the number of filter plates, since reliable hermeticity of the filter plate assembly, due to deformations arising therein in the course of compression, is assured when no less than 70 to 80 filter plates are available.
These deformations are developed as a consequence of lumps of filter cake tending to adhere to the lower portion of the filter plates during cake take off, as well as due to flaws in the manufacture of the filter plates and their seals.
There is further known a filter press which bears the closest resemblance to the filter press of the present invention. This prior art filter press comprises a frame mounted on posts, a thrust plate secured on the posts of the mid-section of the frame, assemblies of filter plates disposed on either side of the thrust plate, and a means for clamping the assemblies of the filter plates includes pressure plates, a drive, and braces by means of which the drive acts on the pressure plates (cf., Japanese Application No. 54-2573, published Jan. 10, 1979).
In this filter press the braces arranged in line with the axis of the filter press on either side thereof secured on the posts of the frame make up up an integral part therewith. The filter plate assemblies are installed on the braces to be capable of longitudinal displacement. In addition, two drives are provided in the above filter press for clamping the filter plate assemblies.
Thanks to the above arrangement of the filter press, the filtering surface is increased doublefold with the result that the specific capacity of the filter press per unit floor area occupied is enhanced whereas the capital outlays and operating costs per unit area of the filtering surface are reduced.
However, because each assembly of the filter plates may contain a definite number of such plates (70 to 80 plates), a further increase in the filtering surface through placing more filter plates is impossible due to deformation taking place during their compression caused by the lumps of filter cake tending to adhere to the lower portion of the filter plates, as well as due to the rigid connection between the drive for clamping the filter plate assemblies and the frame braces.
In addition, for ensuring a reliable operation of the above filter press it is necessary that the two drives of the means for clamping the filter plate assemblies operate synchronously, otherwise the thrust plate has to be much too bulky, which increases the amount of metal required for making the filter press. It is also to be noted that inherent in the aforedescribed filter press construction is a rather overcomplicated system of controlling the synchronous operation of the drives, and excessive specific metal consumption per unit area of the filtering surface resulting in increased capital outlays and operating costs.
Further, mounting of the filter plates directly on the frame braces prevents using in this filter press of known means for automatically taking off the filter cake, which in turn results in greater time and labour expenditures to remove the filter cake. This is disadvantageous, since it affects the specific capacity of the press filter in general.